In most neurosurgical and cranial operations, it is necessary to open a large access hole in the skull by forming a segment of the skull, called a bone flap, which is then bent out of the way or broken out from the surrounding skull. To form the bone flap, several holes are typically drilled through the skull, commonly referred to as burr holes. The burr holes are then connected by osteotomy cuts, for example using a Gigli flexible saw which is passed internally between the burr holes. The saw is then oscillated back and forth to cut the skull along a line connecting adjoining burr holes. The position, number, and size of the burr holes drilled through the skull, as well as the number of connecting osteotomies, is determined by the size, location and geometrical form of the desired bone flap and corresponding access hole. For example, if the bone flap to be removed is a triangular skull cap segment, three holes are preferably drilled at the corner points of the bone flap, connecting osteotomies are made along the sides of the curved triangle delineated by burr holes, resulting in a triangular segment bone flap. The bone flap is subsequently lifted off the underlying dura mater to expose the brain for the further steps of the operation. The bone flap may either be completely removed from the surgical site, or folded back along an uncut edge.
At the end of the procedure, the previously removed bone flap or flaps are repositioned into their original locations, or in different desired locations, relative to the surrounding bone portions. This is typically accomplished in the prior art by drilling small pairs of holes in the surrounding skull bone in several places around the edge of the bone flap. Wire is then carefully threaded through the holes, taking care not to tear the dural tissue covering the brain, then twisted together to secure the edges, the ends tucked into the cut opening so that they do not puncture the skin, and the skin then stitched into place over the skull flap. The procedure is complex and time consuming, and there always is the possibility of injuring the dura either by using the high speed drills that are necessary to form the small holes or by the sharp points of the wire engaging the dura.
Other known methods for providing fixation between adjacent bone portions have included the use of metallic plates of varying configurations which are secured across osteotomies or fracture sites by metallic bone screws. Other devices, such as intramedullary nails or externally fixed pins, have also been used to reduce bone fracture mobility and to improve the relative position of adjacent segments. See for instance U.S. Pat. No. 5,669,912 to Spetzler. The aim of fixation of adjacent bone portions is to immobilize the fracture or osteotomy sites in order to promote localized bone growth in the natural repair of the separation.
A brief survey of prior art methods may be found by looking at patents previously issued on the subject. For instance, U.S. Pat. No. 5,201,737 discloses a flexible plate having a plurality of vanes with holes for receiving bone screws. The plate is placed over a cranial burr hole and adjoining osteotomy lines to provide external fixation of the bone flap to the surrounding cranium. Other external bone plates are shown in U.S. Pat. Nos. 4,651,724; 4,923,471; 5,139,497; 5,372,498; and 5,578,036. All of these plates are designed for external application to fractured bones and require placement of a plurality of screws through the plates and into the bone. Placement of multiple screws through the plates is time consuming, induces additional trauma in drilling the pilot holes for the screws, and may predispose the site to infection.
Other fixation devices are also known, such as the device shown in U.S. Pat. No. 2,511,051 which involves screwing an externally threaded stud into an internally threaded shank. Movement of the stud into the shank is guided by an hexagonal wrench that is inserted through the shank into a countersunk receptacle on the tip of the threaded stud. A similar device is shown in U.S. Pat. No. 5,707,373. These devices have proved cumbersome to use. Further, these devices necessarily require that a portion of the fastener be disposed on both the inner and outer surfaces of the bone, thereby exposing the dura matter of the brain to direct intimate contact with the fastener.
In spite of the use of a variety of fasteners in surgical procedures, improved techniques are still being sought to secure adjacent portions of bone for healing, particularly for securing bone flaps to the surrounding cranium following a craniotomy.